The proposed study is designed to determine the maximum benefit to bone mineral density of the hip that can be derived from increasing intakes of calcium and vitamin D in an elderly population to levels which are expected to be both nutritionally optimal and safe. Average intakes of vitamin D and calcium in those over age 65 are only half to two-thirds of recommended amounts. This study will define what proportion of age-related bone loss is attributable to inadequate intakes of these two nutrients and will provide reference data against which the effectiveness of more invasive or experimental interventions can be assessed. Compared to hormonal and drug therapies, nutritional intervention has few expected side effects, is inexpensive, and can be appropriate for a wide cross-section of the elderly population. The proposed study will determine the effect of daily supplementation with 500 mg of elemental calcium (as citrate malate) plus 700 IU of cholecalciferol on changes in bone mineral density of the hip in 424 men and women who are 65 or older. A three-year randomized, double-blind trial is proposed. One half of the participants will be assigned to the treatment group (calcium+D) and the other half to a placebo group. The two groups will be balanced for sex and age decade. Bone mineral density of the femoral neck, lateral spine and whole body will be measured semi-annually by dual-energy absorptiometry. Other data to be collected include results of blood and urine tests and measures of dietary calcium, dietary vitamin D, grip strength, physical activity, sunlight exposure and travel to other latitudes. Information about smoking and alcohol use, travel, medication use, ethnic background and the occurrence of falls will be obtained by interview. Measurement intervals are planned so that seasonal variation in rates of bone loss and other variables will not confound the analyses or interpretation. In addition to providing an estimate of the maximum benefit to bone mineral density that can be derived from the provision of adequate calcium and vitamin D, this study will examine seasonal patterns of bone loss, overall and seasonal relationships between bone loss and calcium regulating hormones, and influences of biochemical, physiological and behavioral factors on the occurrence of falls.